Distillation is a process in which a liquid and a vapor mixture of two or more substances is separated into its component fractions of desired purity, by the application and removal of heat. Distillation columns are designed to achieve this separation efficiently. One type of column which is used in distillation is a tray column where a number of trays of various designs are used to support the liquid to provide better contact between process vapor and liquid which leads to better separation between the substances into their respective components.
Basic distillation begins from a top of a column to the bottom of the column. Heavier process liquid flows down the column while lighter process vapor ascends up the column. The main components of distillation columns include a vertical shell where the separation of the process vapor and liquid substances occurs, and column internals comprised of fractionation trays alone or in combination with other devices such as structured packing. The column internals increase and enhance the separation between the process vapor and liquid. The internal configurations of the column internals such as tray spacing, column diameter, placement of assemblies to enhance flow increase the efficiency and thereby lead to a lesser requirement of energy.
In traditional mass transfer exchange system, the separation of fluids was often inefficient and expensive. This was due to the differences between the substances and the enormous amounts of energy required to begin the process. For example, a substantially clear and heavier process liquid enters the top of the distillation column and on to a perforated fractionation tray. The process liquid flows across the tray and over a downcomer and downcomer region to a tray inlet portion below. The process fluid at this point is highly dense and can cause seepage through the perforations at the inlet portion of the fractionation tray. This seepage is typically called weeping and leads to a higher energy requirement and lower efficiency in the distillation process. Those skilled in the art will recognize that weeping at the inlet portion is much less tolerable than that at a discharge portion before the liquid enters the downcomer because the process liquid is less active at the inlet portion.
To prevent weeping and increase the activity of the heavier process fluid, the fractionation trays are designed for the pressure drop through numerous perforations across the tray surface to counterbalance the inlet liquid head. Previous fractionation trays incorporated various designs which attempted to prevent the weeping while increasing the activity of the process fluid by deflecting the dense liquid over the initial tray inlet portion perforations of the tray. These devices were commonly referred to as bubble promoters.
Bubble promoters of prior art fractionation trays included sloped and box-like structures. U.S. Pat. No. 3,282,576 to Bruckert discloses a sloped surface at the tray inlet area. The Bruckert device comprises a "roof-like" structure with an imperforate ascending portion which receives the dense process liquid and a perforated descending portion where lighter process vapor from below the tray ascends through the perforation. However, under high process vapor flows, the vapor streams can shoot through the heavier liquid and hit the underside of the tray above. This results in entrainment where the liquid is carried by the vapor to hit the bottom of the tray above. This reduces tray efficiency and can cause contamination of the substances. Excessive entrainment can lead to flooding of the tray section where the increased pressure from the excessive vapor flow can force the heavier fluid into the downcomer region which backs up the flow of liquid onto the tray above. This affects the capacity of the entire column and increases the energy requirements of the system thus lowering efficiency. The Bruckert device also accelerates the process vapor into the process liquid which increases the flow velocity over the fractionation tray surface. Any unnecessary acceleration will cause a maldistribution of fluid across the fractionation tray which changes flow patterns in an undesirable manner and lowers the efficiency of the separation process.
Another prior art bubble promoter is disclosed in U.S. Pat. No. 3,700,216 to Uitti et al. The Uitti device employs a sloped imperforate surface as in the Bruckert device but comprises a vertical drop and opening at the downstream end from the tray inlet region. However, the flow of vapor from below the tray is excessively aerated and substantially forced across the tray surface. This is typically called a spraying effect and interferes with the normal interaction of process vapor and liquid across the fractionation tray by changing the flow pattern. The aeration of the liquid also prevents interaction between the vapor and liquid at the inlet portion of the tray which is critical to the efficiency of the tray and column.
One attempt to control the aeration at the tray inlet portion and spraying is disclosed in U.S. Pat. No. 4,275,021 to Kirkpatrick et al. The device disclosed in Kirkpatrick comprises an adaptation of part of the inlet of the fractionation tray to have two imperforate wall members separated by an intermediate perforated wall member. While the Kirkpatrick apparatus reduces the amount of horizontal jetting or spraying of the vapor across the fractionation tray surface, a new problem is introduced whereby the perforations of the intermediate member can flood the tray inlet region by shooting upward under high vapor pressures. The higher vapor pressure will back the denser process fluid into the downcomer and onto the tray above. Additionally, under lower vapor pressure, the process fluid flowing from the downcomer will not be activated as the denser and heavier fluid flows over the intermediate perforated wall member thus preventing the dense process liquid from becoming active and lowering the efficiency of the tray and column.
Accordingly, there is a need for a tray inlet apparatus which activates the denser and heavier process fluid while allowing interaction with the lighter process vapor. The activation of the process fluid occurs while eliminating any weeping or flooding in the tray inlet area and spraying acceleration across the fractionation tray surface.